1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the device.
2. Description of the Related Art
A liquid crystal display (LCD) is a flat-panel display that has a number of advantageous features including high resolution, drastically reduced thickness and weight, and low power dissipation. The LCD market has been rapidly expanding recently as a result of tremendous improvements in its display performance, significant increases in its productivity, and a noticeable rise in its cost effectiveness over competing technologies.
A twisted-nematic (TN) mode liquid crystal display device, which used to be used extensively in the past, is subjected to an alignment treatment such that the major axes of its liquid crystal molecules, exhibiting positive dielectric anisotropy, are substantially parallel to the respective principal surfaces of upper and lower substrates and are twisted by about 90 degrees in the thickness direction of the liquid crystal layer between the upper and lower substrates. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules change their orientation direction into a direction that is parallel to the electric field applied. As a result, the twisted orientation disappears. The TN mode liquid crystal display device utilizes variation in the optical rotatory characteristic of its liquid crystal layer due to the change of orientation directions of the liquid crystal molecules in response to the voltage applied, thereby controlling the quantity of light transmitted.
The TN mode liquid crystal display device allows a broad enough manufacturing margin and achieves a high productivity. However, the display performance (e.g., the viewing angle characteristic, in particular) thereof is not fully satisfactory. More specifically, when an image on the screen of the TN mode liquid crystal display device is viewed obliquely, the contrast ratio of the image decreases significantly. In that case, even an image, of which the grayscales ranging from black to white are clearly observable when the image is watched straightforward, loses much of the difference in brightness between those grayscales when viewed obliquely. Furthermore, the grayscale characteristic of the image being displayed thereon may sometimes invert itself. That is to say, a portion of an image, which looks darker when viewed straight, may look brighter when viewed obliquely. This is a so-called “grayscale inversion phenomenon”.
To improve the viewing angle characteristic of such a TN mode liquid crystal display device, an inplane switching (IPS) mode liquid crystal display device (see Japanese Patent Gazette for Opposition No. 63-21907), a multi-domain vertical aligned (MVA) mode liquid crystal display device (see Japanese Laid-Open Publication No. 11-242225), an axisymmetric aligned (ASM) mode liquid crystal display device (see Japanese Laid-Open Publication No. 10-186330), and a liquid crystal display device disclosed in Japanese Laid-Open Publication No. 2002-55343 were developed recently.
All of these were developed relatively recently as TN mode liquid crystal display devices with improved viewing angle characteristics. In a liquid crystal display device operating in each of these newly developed wide viewing angle modes, even when an image on the screen is viewed obliquely, the contrast ratio never decreases significantly or the grayscales never invert unlike the old-fashioned TN mode liquid crystal display devices.
However, in the IPS or MVA mode liquid crystal display device, the gray-scale voltage applied to the liquid crystal layer needs to be controlled more precisely than in the conventional TN mode liquid crystal display device. This is because in the IPS or MVA mode liquid crystal display device, the ratio α of the variation in luminance Y to the variation in applied voltage V (i.e., α=ΔY/ΔV) is greater than in the TN mode LCD.
Another reason is that the TN mode liquid crystal display device usually conducts a display operation in normally white (NW) mode whereas the IPS or MVA mode liquid crystal display device needs to perform a display operation in normally black (NB) mode.
In a normal display device with 256 grayscales (in which the grayscale 0 represents the lowest brightness (i.e., black) and the grayscale 255 represents the highest brightness (i.e., white)) and with its γ characteristic controlled at 2.2, unevenness of display (i.e., unevenness in brightness) is observed most remarkably when a color between grayscales 20 and 60 (i.e., a intermediate tone (gray) near black) is displayed. In an NB mode liquid crystal display device, the ratio α of the variation in luminance to the variation in applied voltage at such an intermediate tone near black is greater than that of an NW mode liquid crystal display device. For that reason, to reduce the unevenness of display, the voltage applied to the liquid crystal layer needs to be controlled highly precisely.
Accordingly, in the IPS or MVA mode liquid crystal display device, the patterning accuracy of TFTs and other circuit components and the performance of driver circuits (including various signal voltage generators) must be increased, thus raising the manufacturing cost significantly. Stated otherwise, if the patterning accuracy of TFTs and other circuit components and the performance of driver circuits were almost the same, the IPS or MVA mode liquid crystal display device would exhibit lower uniformity (or display quality) and lower resolution than the conventional TN mode liquid crystal display device when the image on the screen is viewed straightforward.
As described above, the unevenness of display due to the high luminance-applied voltage variation ratio (α=ΔY/ΔV) is much more significant in the IPS or MVA mode liquid crystal display device than in the conventional TN mode liquid crystal display device (and more significant in an NB mode liquid crystal display device than in an NW liquid crystal display device). Nevertheless, this problem is commonly observed in every liquid crystal display device, although their degrees are different. And if that ratio of the variation in luminance to the variation in applied voltage (i.e., α=ΔY/ΔV) can be reduced, the display quality can be improved in any liquid crystal display device operating in any mode.